The parts of a jet engine are primarily formed from superalloys which when used in the high temperature areas of an engine have a hard protective exterior coating to prevent oxidation of these parts. In the manufacture and subsequent operation of jet engines, microcracks can form in the engine parts. Oxides including aluminum oxide, chromium oxide, and titanium dioxide form on the untreated surfaces of these cracks. These oxides are normally not reducible in a standard vacuum or hydrogen thermal cycle and prevent repair of the surface since they prevent deposition of surface treatment compositions.
There are three primary methods currently used to remove the contaminant oxides. One method is a two-step reaction involving elemental chrome and ammonium fluoride heated in the presence of hydrogen to generate hydrogen fluoride gas. This generally requires treatment of the entire product. The second method uses the thermal decomposition of polytetrafluoroethylene in the presence of dry hydrogen to produce hydrogen fluoride gas. The third method is hydrogen fluoride gas treatment itself introduced to the cleaning retort.
The treatment of the entire surface with hydrogen fluoride gas is effective but requires treatment of the entire article which then must receive post-treatment, if previously coated. This is very costly. Localized treatment of the crack area is definitely preferred if accomplished effectively.